RFC 4824 IP over SFSS April 2007
1. Introduction
This document specifies IP-SFS, a method for the encapsulation and
transmission of IPv4/IPv6 packets over the Semaphore Flag Signaling
System (SFSS). The SFSS is an internationally recognized alphabetic
communication system based upon the waving of a pair of hand-held
flags [JCroft, Wikipedia]. Under the SFSS, each alphabetic character
or control signal is indicated by a particular flag pattern, called a
Semaphore Flag Signal (SFS).
IP-SFS provides reliable transmission of IP datagrams over a half-
duplex channel between two interfaces. At the physical layer, SFSS
uses optical transmission, normally through the atmosphere using
solar illumination and line-of-sight photonics. A control protocol
(Section 4) allows each interface to contend for transmission on the
common channel.
This specification defines only unicast transmission. Broadcast is
theoretically possible, but there are some physical restrictions on
channel direction dispersion. This is a topic for future study.
The diagram in Figure 1 illustrates the place of the SFSS in the
Internet protocol hierarchy.
+-----+ +-----+ +-----+
| TCP | | UDP | ... | | Host Layer
+-----+ +-----+ +-----+
| | |
+-------------------------------+
| Internet Protocol & ICMP | Internet Layer
+-------------------------------+
|
+-------------------------------+
| SFSS | Link Layer
+-------------------------------+
Figure 1: Protocol Relationships
2. Definitions
Link: A link consists of two (2) interfaces that share a common
subnet.
Link Partner:
The opposite interface.
Session: The transmission of an entire IP datagram.
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RFC 4824 IP over SFSS April 2007
SFS: One Semaphore Flag Signal, i.e., one flag pattern (Section
3.2).
SFSS: The Semaphore Flag Signaling System.
IP-SFS: IP over Semaphore Flag Signaling System.
3. Protocol Discussion
IP-SFS adapts the standard SFSS to encode an alphabet of 16 signals
(flag patterns) to represent data values 0-15 (Section 3.2.1) and 9
signals to represent control functions (Section 3.2.2). With 16 data
signals, IP-SFS transmission is based upon 4-bit nibbles, two per
octet. Each of the signal patterns defined in Section 3.2 is called
an SFS.
3.1. IP-SFS Frame Description
IP datagrams are formatted into IP-SFS frames by adding IP-SFS
headers and trailers. Figure 2 shows the format of one IP-SFS frame.
The frame is delimited by a control SFS called FST (Frame Start) and
a control SFS called FEN (Frame End). It is composed of a series of
4-bit nibbles, one per SFS.
An IP datagram will be fragmented into multiple successive IP-SFS
frames if necessary. When an IP datagram is fragmented into N
frames, the first frame will be sent with frame number N-1, the
second with frame number N-2, ..., and the last with frame number 0.
0 1 2 3
+--------+--------+--------+--------+--------+
| FST |Protocol|CksumTyp|Frame No|Frame No|
+--------+--------+--------+--------+--------+
| |
// DATA Payload //
| |
+--------+--------+--------+--------+---------+
| CRC | CRC | CRC | CRC | FEN |
+--------+--------+--------+--------+---------+
Note that each field represents one SFS or 4 bits.
Figure 2: IP-SFS Frame Format
FST: Frame Start control SFS
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RFC 4824 IP over SFSS April 2007
FUN: Frame UNdo. As long as Frame ENd is not sent, the transmitter
or the receiver may send a FUN to restart the transmission of
the current frame. This signal will be unacknowledged and may
be ignored by the receiver.
ACK: Frame ACK. Acknowledges reception of one frame.
KAL: KeepALive. Keep a connection alive. Is to be transmitted in
State Idle at a frequency of at least KAL_FREQ (see
Section 4.2). This signal will be unacknowledged.
NAK: Frame No AcK. The frame received is incorrect.
RTR: Ready To Receive. Receiver acknowledges it is ready to receive.
RTT: Ready To Transmit. Sender requests permission to initiate
transmission.
3.5. Protocol Limitations
Due to the physical characteristics of the transfer channel, bit
error rates are expected to be in the range of 1e-3 (boy scout) to
1e-4 (professional sailor), and also depend a number of physical
factors. Poor visibility due to weather conditions or lack of
illumination (e.g., night time) can drastically increase the error
rate.
IP-SFS provides no means to handle frame reordering or dual
(multiple) frame reception. Thus, the protocol is not suitable in
environments where interfaces are moving fast and/or when the path of
light is long.
3.6. Implementation Limitations
Maximum payload per frame: 510 SFS (0...510) nibbles (0 to 255
octets)
Maximum SFS per frame: 518
Maximum frames per session: 255 (0...254)
4. Interface Discussion
An interface is constructed through the participation of one or more
humans. A knowledge of the SFSS is recommended, but its absence can
be compensated by a well-designed GUI.
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RFC 4824 IP over SFSS April 2007
4.1. Data Link Control
This section describes the control protocol used to allocate the
half-duplex data link to either interface.
Interfaces know three States: Idle, Transmitting (TX), and Receiving
(RX).
4.2. Establishing a Connection
IP-SFS is strictly point-to-point. Unless interface members are
acquainted with each other, a brief introduction of both sides is
suggested prior to setting up a link to reduce the likelihood of
interface-spoofing attacks.
Interfaces must agree on two different IP addresses on the same
subnet.
Interfaces are free to negotiate any period of time as TIMEOUT.
Possible values for TIMEOUT are the time it takes to smoke a
cigarette or the time it takes to drink a glass of water. If
negotiation fails, TIMEOUT defaults to 60 seconds.
The same procedure may be applied for the KeepALive FReQuency
(KAL_FRQ). The period of KAL_FRQ (1/KAL_FRQ) should be at least
5*TIMEOUT.
4.3. State Idle
Interfaces in State Idle must be ready to send an IP datagram
provided by a local higher-level protocol or to receive a datagram
from the link-partner. Interfaces in State Idle must send keep-alive
signals KAL at a frequency of at least KAL_FRQ.
There are no further definitions for State Idle, but we recommend
staying away from alcoholic beverages or other types of drugs that
could lead to an increased number of FUN and/or SUN signals, a
decrease in bandwidth, or an increase of line latency.
If the number of IP datagrams in the transmission queue is >0, the
interface may try to initiate a session by sending an RTT to the link
partner. If the link partner ready to receive, it returns an RTR
signal.
4.4. Session Initiation
An interface receiving a datagram from an Internet layer protocol
level may start signaling RTT.
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RFC 4824 IP over SFSS April 2007
If the link partner does not respond with RTR within TIMEOUT, or the
link partner responds with RTT, the interface switches to State Idle
for a random period of time between 2 seconds and TIMEOUT, before
resending the RTT.
If the link partner transmits RTR, the interface transmits the number
of IP-SFS frames to be transmitted in this session as two SFSs
followed by another RTT. If the link partner does not transmit the
same number of IP-SFS frames followed by RTR within 3*TIMEOUT, the
interface switches to State Idle.
If the link partner transmits the same number of IP-SFS frames
followed by RTR, the interface switches to State Transmitting.
Unless obstructed through environmental noise or great distance,
hollering can be used to request line discipline from the link
partner in State Idle. The use of cellphones is also an option,
whereas throwing objects or using guns is not recommended, since it
could result in interface damage or destruction. This would be
counterproductive.
4.5. State Transmitting
Transmission of one IP-SFS frame starts with FST. After FST and
before FEN have been transmitted, the interface may acknowledge a
received FUN and restart the transmission of the active frame or
discard the signal and continue transmission of the active IP-SFS
frame.
If an error occurs by transmitting a wrong data SFS, the interface
may invalidate the last data SFS by transmitting SUN followed by the
correct signal. A series of incorrectly transmitted data SFSs may be
invalidated by sending a SUN for each invalid SFS, effectively back-
spacing the sequence.
Control SFSs cannot be invalidated.
If an error occurs, the interface may also transmit FUN and restart
transmission of the active IP-SFS frame.
Whether the interfaces choose SUN or FUN for error correction is up
to the interface, but it is suggested to use SUN for a single invalid
SFS, and FUN whenever the interface failed to transmit several SFSs
in a row.
After FEN has been transmitted, the transmitting interface waits for
the link partner to transmit ACK or NAK.
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RFC 4824 IP over SFSS April 2007
If ACK has been received, the transmitting interface removes the
active frame and starts transmitting the next IP-SFS frame. If no
frames are left, the interface switches to State Idle.
If NAK has been received, the transmission failed, and the interface
must start transmitting the active frame again.
If the link partner does not transmit ACK or NAK within TIMEOUT, the
transmission failed, and the interface must start retransmitting the
active IP-SFS frame.
If transmission of the same IP-SFS frame fails 5 times, the interface
leaves the IP datagram in the transmission queue and switches to
State Idle.
4.6. State Receiving
In State Receiving, the interface stores each SFS received from the
link partner in the receiving queue in the order of appearance.
After FST and before FEN have been received, the interface may
transmit FUN at any time to request a retransmission of the entire
IP-SFS frame.
If the time between two received SFSs exceeds TIMEOUT, the receiving
interface must discard all data from the active IP-SFS frame and may
additionally transmit FUN. If the link partner does not continue
transmitting within a second TIMEOUT period, the interface must clear
the receiving queue and switch to State Idle.
If the interface receives SUN from the link partner, it must discard
the last received data SFS (if any). If N SUNs are received in a
row, then the last N data SFS must be discarded, unless there are no
more data SFS in the frame. If there are no more data SFS in the
frame to be discarded, the SUN signal must be ignored by the
interface.
If the receiving interface receives FUN from the link partner, it is
free to discard the frame received thus far. We suggest honoring FUN
since discarding the signal will decrease bandwidth.
After FEN has been received, the receiving interface evaluates the
checksum. If the checksum is good, the interface transmits ACK. If
the Frame Number of the active frame is 0, the interface passes the
entire data from the receiving queue to the higher level protocol,
clears the receiving queue, and switches to State Idle.
If the checksum is incorrect, the interface transmits NAK.
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RFC 4824 IP over SFSS April 2007
4.7. Terminating a Connection
If the interface is in State Idle and the link partner did not
transmit any kind of SFS for at least five times 1/KAL_FRQ, the
connection is terminated and the interfaces are free to disband.
4.8. Further Remarks
Interfaces are connected to computer hardware by means of a suitable
input device (RX) and a suitable output device (TX). Possible
devices include keyboard, mouse, and monitor. Other means of
connection are subject to availability and budget.
Although it is theoretically possible to combine the TX and RX parts
of an interface in one human being, we suggest dividing the
operations among at least two people per interface. For longer
transmissions (multimedia streaming, video conferencing, etc.),
consider rotating and providing substitutes.
Bandwidth tends to vary. Typically TX starts at about 2-4 bits/s and
will decrease over time due to exhaustion and lack of concentration.
When an interface in TX State signals at a rate higher than the RX
interface is able to receive, signal loss occurs.
5. Security Considerations
By its nature of line-of-sight signaling, IP-SFS is considered
insecure. The transmission of sensitive data over IP-SFS is strongly
discouraged unless security is provided by higher level protocols.
Interfaces tend to keep data in their memory. There is no way to
determine the lifetime of data in memory. As a side effect, data
might show up in unwanted locations at undesired times.
We are currently not aware of a technique to reliably delete data
from interfaces' memory without permanent interface destruction.
6.Acknowledgements
We thank Eva Ursprung and Doris Jauk-Hinz from Womyn's Art Support
(WAS), Anita Hofer, Reni Hofmueller, Ulla Klopf, Nicole Pruckermayr,
Manfred Rittler, Martin Schitter, and Bob Braden for all their
contributions and support of this project.
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RFC 4824 IP over SFSS April 2007
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